A diamond is expected to be an ultimate semiconductor substrate. This is because a diamond has a lot of excellent characteristics, which are unparalleled in anywhere as a semiconductor material, such as high thermal conductivity, high electron/hole mobility, high dielectric breakdown field strength, a low dielectric loss, and a wide bandgap. The bandgap thereof is about 5.5 eV which is a very high value in existing semiconductor materials. Particularly, in recent years, UV light emitting elements using a wide bandgap, field effect transistors having excellent high-frequency characteristics, and the like have been developed.
When the diamond is employed as a semiconductor substrate, a flat substrate having no curvature (that is, no warp) in appearance is preferable in that axes of crystals have no inclination. In order to obtain such a diamond substrate, RAF growth method used for the SiC single-crystal growth has to be applied at the current time. The RAF growth method is a method of repeatedly growing SiC single crystals in the a-face orientation and is called Repeated a-Face (RAF) method. After an a-face single crystal is cut out from the grown ingot, and crystal is grown from the cut a-face, the cutting of a-face single crystal and the growth from the cut a-face are repeated. Then, a seed crystal is cut out from the ingot. The diamond substrate obtained using the RAF method has a size of approximately ten mm square at maximum at the current time.
When it is considered that a diamond is applied to a semiconductor, a certain size such as a diameter of several inches is required. This is because, when a processing device used for micro-machining of a general semiconductor such as Si is applied to the diamond, it is difficult to use the processing device for a small-sized substrate smaller than several inches.
Some ideas have been proposed as a method of causing a diamond having a certain size to grow. Out of the methods, there are several highlighted candidate methods such as a diamond single crystal growth method (so-called mosaic growth method; for example, see Patent Literature 1) of arranging a plurality of small diamond single-crystal substrates or a manufacturing method of forming a diamond film through a heteroepitaxial growth method on a base substrate by using a single-crystal magnesium oxide (MgO) substrate as the base substrate (for example, see Patent Literature 2).
The mosaic growth method is a technique of growing and forming a large-sized diamond single-crystal substrate by arranging a plurality of diamond single-crystal substrates in a shape of tiles and causing diamond single crystals to newly grow on the diamond single-crystal substrates through a homoepitaxial growth method. However, on the boundaries between the diamond single-crystal substrates arranged in the shape of tiles, coupling boundaries are generated and deteriorate the crystal quality. Therefore, the coupling boundaries are necessarily generated in the diamond single crystals obtained through the mosaic growth method.
The reason of formation of coupling boundary is that diamond single crystals grow randomly in the area of the coupling boundary, coalescence occurs from various directions, and a lot of dislocation are generated in the coupling boundary. This coupling boundary is a distinct boundary line which can be observed visually.
Since the part of the coupling boundary cannot be used for growth of a semiconductor device, a practically usable area is limited with respect to the area of the diamond single-crystal substrate obtained through the mosaic growth method.
To make matters worse, the area of the diamond single-crystal substrate that can be used to manufacture a semiconductor device does not necessarily match the size of a semiconductor device chip. Therefore, in a process of manufacturing a semiconductor device in such a diamond single-crystal substrate, it is necessary to perform the process to avoid the coupling boundary. Accordingly, the process of manufacturing a semiconductor device becomes complicated.
On the other hand, the heteroepitaxial growth method is a technique of causing a diamond film which will be a diamond substrate to epitaxial-grow on a base substrate formed of a material having different physical properties. Since a single diamond film epitaxial-grow on a single base substrate, there is no need to worry about a coupling boundary generated between a plurality of diamond single-crystal substrates unlike the mosaic growth method.
Therefore, out of the two techniques including the mosaic growth method and the heteroepitaxial growth method, the heteroepitaxial growth method is particularly prospective in that the area of the substrate used to manufacture a semiconductor device is not easily restricted.
However, a stress is generated in crystals of the grown diamond substrate due to a lattice constant difference and a thermal expansion coefficient difference between the base substrate and the diamond and thus a warp or a crack is generated in the diamond substrate. Accordingly, it is not easy to obtain a large-sized substrate even using the heteroepitaxial growth method.
In this regard, several techniques of the related art relevant to a decrease in the stress generated in a diamond formed through the heteroepitaxial growth method have been reported (for example, see Patent Literature 3).
Patent Literature 1: Japanese Patent No. 3387154
Patent Literature 2: Japanese Patent No. 5066651
Patent Literature 3: Japanese Unexamined Patent Application Publication No. 2007-287771